I think it's different for nextgen and classic. For nextgen, I believe it's #0f1c2b, and for classic, I believe it's #0d253e
If interested, the way I check them is by taking a screenshot, opening the image in photoshop (or any other image editing program for that matter), and using the color picker tool to check the color. http://i.imgur.com/3CgNH.png
Are you looking to make signatures with the background sythe's colour? I'd say just stick to transparency, it works best with anything and if people are on other forum styles then the sig doesn't stick out like dogs balls.